1. Field of the Invention
The present invention relates to a system for inspecting pattern defects of printed wiring boards, and more particularly, it relates to a method of and a device for automatically detecting defects present in wiring patterns by comparing pattern data of the printed wiring boards themselves with each oher.
2. Description of the Prior Art
Pattern defect inspection of printed wiring boards (hereinafter referred to as PWBs) is directed to recognize defects in wiring patterns to correct the same or abandon defective PWBs thereby to efficiently produce PWBs of good quality. Such inspection is generally performed by human inspectors, the operation for which requires much labor and is inferior in workability.
Recently with remarkable development in application of functional elements to ICs and LSIs, PWBs have been rapidly subjected to high density and high accuracy wherefore high reliability is required. Particularly in multilayer PWBs, employment of which is rapidly increased by high densification, pattern defect inspection of inner layers during manufacturing steps is more important than inspection of completed PWBs. Since each PWBs are remarkably highly densified, a great number of steps are required for visually inspecting the same. Thus, there have generally been attempts for optical and electronical automatic inspection, and such attempts are currently put into practice.
Systems of such automatic inspection generally include a system of extracting features of patterns employing information on forms of wiring patterns to be inspected and a comparing system of comparing patterns thereby to find difference therebetween. The former system can detect defects from the patterns themselves, whereas it requires specific defect detecting algorithm based on pattern recognition technique since the information on the forms is employed. The latter system is relatively technically simple in developing and designing stages since it requires no specific algorithm, whereby the cost for development thereof is reduced.
In a pattern defect inspection device of the comparing system, reference patterns are necessarily set so that patterns of PWBs to be inspected are compared with the same thereby to detect the defects by difference in the results of comparison.
The reference patterns are set in two types of methods, one employing original information in design of the PWBs by CAD as the reference patterns and the other employing a previously selected PWB as the reference pattern.
The former method necessarily requires a memory unit for storing the original patterns as the design information, whereby the memory unit requires great volume of storage capacity. Further, the design data are required to be temporarily converted into raster type data for comparison since the same are generally prepared as vector type data. Such conversion requires considerable work, materials such as exclusive hardware and exclusive operators. Further, in order to process the operation by software, elements such as a high-speed computer, disc unit and magnetic tape are required, leading to increased cost with problems rising in processing speed. Further, inner layers of multilayer PWBs, particularly those for industrial use are produced in various types and small amunts, e.g., in 5 to 20 for one type and hence workability is relatively inferior for the labor. In addition, much labor is required for control and application of the reference patterns.
The latter method of selecting the reference PWB processes a non-defective or defect-corrected flawless wiring pattern as an absolute reference, wherefore only one PWB is previously selected by visual inspection. However, since the PWBs, particularly inner layers thereof are produced only in units of 5 to 20 as hereinabove described, it is considerably disadvantageous in workability to select a relatively flawless one from the PWBs by visual inspection, in view of the ratio of the visual inspection time to the number of the entire PWBs.
Pattern defect inspection in the layer system generally adopts structure provided with two pattern reading units in parallel, so as to read the reference PWB pattern by one reading unit and read the pattern of a PWB to be inspected by the other reading unit, thereby to compare the pattern data with each other for deciding defects by difference therebetween. However, the system structure is enlarged in this case since the two pattern reading units are provided in a parallel manner. Further, the most difficult point of this method resides in that the reference pattern and the pattern to be inspected must be correctly aligned with each other. In order to mechanically align the PWBs, employed are conventional mechanical aligning means such as positioning pins and margins. However, such mechanical aligning means cannot avoid misalignment of about 0.1 to 0.5 mm. It is impossible to effect detection on defects of 10 to 50 mm by comparing 10 mm order pixels in pixel correspondence only by mechanical alignment of the PWBs. Therefore, such an attempt has generally been adopted to partially display patterns of two PWBs on a monitor screen so that a skilful operator reads misregistration in the patterns thereby to finely move one of the pattern reading units in order to align corresponding pixels of the PWBs. However, inspection efficiency is limited in such an attempt since such alignment must be effected for inspection of every PWB. Further, the PWBs are manually placed on an inspection table in a general inspection device of this type, and improvements are desired in this point in view of workability.
Apart from the aforementioned methods, it has once been considered to process a photomask for producing a PWB as reference. However, a PWB is generally obtained through a chemical process of etching, and the line width of the wiring pattern varies with the etching process or the extent of the etching. When the photomask is employed as the reference pattern, the amount of change in the line width must always be subjected to feedback to reference pattern data, operation for which is remarkably complicated and hence such an attempt has not been practically adopted.
It must be taken in due consideration in pattern inspection that a PWB is generally obtained through the chemical process of etching.